The present invention relates generally to Alternating Current (AC) thermal plasma generators. More particularly, this invention pertains to three-phase, AC thermal plasma generators.
Thermal plasma is generally defined as a state of matter, which exhibits many properties similar to gas, contains substantially equal numbers of positive and negative ions and radicals, and is a good conductor of electricity. Thermal plasma may be created by increasing the internal energy of matter. The internal energy of matter may be increased by exposing matter to an electric arc in such a way that the electrical energy from the electric arc is transferred to the matter.
Different techniques for generating thermal plasma have been researched for many years. As a result, several different types of thermal plasma generation systems have been developed. One such example is the thermal plasma, metal cutting, torch. Each system differs in the way that the electric arc is initiated and sustained.
Thermal plasma has been created using both direct current and alternating current devices. Low power direct current (DC) and inductive coupling (IC) thermal plasma generators are currently used in semiconductor, film deposition, and other high technical applications. Alternating current (AC) thermal plasma generators are ideal for such applications as radioactive materials vitrification, decontamination of pathogenic materials and substances (e.g., hospital waste), and reduction and/or safe decomposition of hazardous waste or difficult to destroy materials. In addition, AC thermal plasma generators may be used in chemical processes that require the heating of materials in the absence of oxygen or to reduce or de-compose waste materials into clean energy fuel.
In all DC arc-generating systems, the arc is initiated between a cathode and an anode. In a transferred arc system, a substance being treated, a molten metal for example, is used as one of the electrodes. In a non-transferred arc system, the electrodes are independent of the treated substance.
AC thermal plasma generators are more efficient and less expensive than DC thermal plasma generators because complicated and expensive rectifier equipment is not necessary. However, AC thermal plasma generators have disadvantages as well.
Single-phase AC thermal plasma generators have been found to be inherently unstable due to the fact that the electric arc is extinguished every half cycle. As a result, the electric arc must be initiated 120 times per second. Three-phase AC thermal plasma generators overcome this instability problem.
A three-phase AC thermal plasma generator is described in U.S. Pat. No. 5,801,489 commonly owned by Applicant. This thermal plasma generator is powered by alternating current directly from a conventional electric utility network or from a generator system. The use of alternating current provides a significant improvement in efficiency over prior art DC plasma generators. In addition, this generator uses a three-phase electrode arrangement to produce a stable arc. This arrangement results in a highly stable arc thereby overcoming the instability found in other prior art AC thermal plasma generators.
One disadvantage of the ""489 patent is the short lifetime of the primary electrodes. The primary electrode arrangement in the ""489 patent provides small arc-working areas on each primary electrode. As a result, the primary electrodes wear out in a short time period and must be replaced. This is a time-consuming and expensive process.
Another disadvantage of the ""489 patent is the fact that the primary electrodes are fixed in place. As the primary electrodes wear down due to the small arc-working areas on each electrode, the gap between the primary electrodes increases. As the gap increases, the voltage necessary to initiate an arc across the gap increases and generator efficiency is reduced.
Still another disadvantage of the ""489 patent is the use of a single pneumatic ring, located adjacent to the primary electrodes, to introduce the working stream into the arcing chamber. As a result, a non-uniform thermal plasma steam is produced, having hot and cold spots, which reduces generator efficiency.
What is needed, then, is an improved three-phase AC thermal plasma generator having an adjustable primary electrode arrangement that results in a substantially constant gap between primary electrodes and large arc-working areas, which increases generator efficiency, extends the life of electrodes, and reduces primary electrode replacement costs. Additionally, what is needed is a means of introducing the working stream into the arcing chamber that results in uniform thermal plasma generation to further increase generator efficiency.
The thermal plasma generator of this invention is powered with alternating current directly from a conventional electric utility network or from a generator system. A significant improvement in efficiency over DC generators is obtained by using alternating current because of reduced losses that would otherwise occur in the power supply. In addition, the process of convective heat-exchange takes place because of the rapid movement of the arcs within the chamber, high turbulence stream flow, and diffusion of the arc inside the chamber. The use of relatively low voltage alternating current eliminates the need for an additional high-voltage direct current power supply thus reducing the cost of fabrication and maintenance.
The primary electrodes are positioned so that a portion of each electrode forms a narrow gap with respect to a portion of each of the other two electrodes, and another portion of each electrode forms a wider gap with respect to another portion of each of the other two electrodes. This configuration results in a larger electrode arc-working area and extends the life of the electrodes.
The use of electrodes configured to provide a large arc-working area and the application of the rail gun effect (the movement of the arc under the influence of its own magnetic field) allows the use of electrodes cooled by water with the operational advantage of several hundreds of hours without maintenance. Two types of electrodes can be used: tubular water-cooled electrodes made of copper and rod electrodes made of tungsten alloy and cooled with stream.
In accordance with one object of the invention, the plasma generator of the present invention utilizes a novel adjustable arrangement and configuration of electrodes that provides a large arc-working area and redistributes thermal load on the electrodes by rapidly moving the arc along the electrodes. The large arc-working area and redistribution extends the life of the electrode. The adjustment mechanism ensures that the gap between each electrode remains constant and, as a result, maintains generator efficiency.
In accordance with another object of the invention, a high quality, uniform plasma is obtained by injecting the working stream into the arcing chamber of the generator through multiple stream rings. The use of multiple rings improves the mixing of the working stream.